This invention pertains to the field of digital imaging, and more particularly to a colorant reduction method used during the process of printing a digital image.
In the field of digital printing, a digital printer receives digital data from a computer and places colorant on a receiver to reproduce the image. A digital printer may use a variety of different technologies to transfer colorant to the page. Some common types of digital printers include inkjet, thermal dye transfer, thermal wax, electrophotographic, and silver halide printers.
Often when printing digital images, undesirable image artifacts may result when an excessive amount of colorant is placed in a small area on the page. These image artifacts degrade the image quality, and can result in an unacceptable print. In the case of an inkjet printer, some examples of these image artifacts include bleeding, cockling, banding, and noise. Bleeding is characterized by an undesirable mixing of colorants along a boundary between printed areas of different colorants. The mixing of the colorants results in poor edge sharpness, which degrades the image quality. Cockling is characterized by a warping or deformation of the receiver that can occur when printing excessive amounts of colorant. In severe cases, the receiver may warp to such an extent as to interfere with the mechanical motions of the printer, potentially causing damage to the printer. Banding refers to unexpected dark or light lines or streaks that appear running across the print, generally oriented along one of the axes of motion of the printer. Noise refers to undesired density or tonal variations that can give the print a grainy appearance, thus degrading the image quality. Although these artifacts are presented in the context of an inkjet printer, it is known to those skilled in the art that similar artifacts commonly exist with the other above mentioned printing technologies also.
In a digital printer, satisfactory density and color reproduction can generally be achieved without using the maximum possible amount of colorant. Therefore, using excessive colorant not only introduces the possibility of the above described image artifacts occurring, but is also a waste of colorant. This is disadvantageous, since the user will get fewer prints from a given quantity of colorant.
It has been recognized in the art that the use of excessive colorant when printing a digital image needs to be avoided. Generally, the amount of colorant needed to cause image artifacts (and therefore be considered excessive) is receiver, colorant, and printer technology dependent.
U.S. Pat. No. 4,930,018 to Chan et a. teaches a method of reducing paper cockle and graininess of inkjet prints utilizing multiple inks with different dye loadings. In this method, a given grey level can be reproduced a variety of different ways, some of which will use more colorant than others. The different ways to reproduce a given grey level are rank ordered according to the total ink coverage, and a selection is made by iterating through the order until one is found that satisfies a specified maximum coverage limit.
U.S. Pat. No. 5,515,479 to Klassen teaches a method for reducing marking material (i.e., ink) coverage in a printing process by determining the ink coverage for each pixel in an image, determining if too much ink will be placed on the page in a given area, and reducing the amount of ink to an acceptable level by turning xe2x80x9coffxe2x80x9d a fraction of the pixels in the given area. The determination of which pixels to turn off is made by using a processing order through each area which tends to randomize the turn off effect. While this method successfully reduces the amount of ink placed on the page in a given area, it can introduce pattern noise into the image because of the processing order method of selecting which pixels to turn off. Also, the pixels that are turned off in each color separation are not correlated, which can lead to a grainy appearance to an image region that should appear otherwise uniform.
U.S. Pat. No. 5,563,985 to Klassen et al. addresses the problem of pattern noise by selecting which pixels to turn off in response to a random number function. While this method successfully eliminates pattern noise that can be generated in a given area, it can introduce random noise into the image because the selection of which pixels to turn off is determined by a random process. While this may be visually less objectionable than pattern noise, it is still not optimal.
It has been demonstrated that the sensitivity of the human visual system decreases with increasing frequency of modulation. Thus, high frequency patterns are less visible to the human eye than low frequency patterns. In the art of digital halftoning, this effect is taken advantage of by designing halftoning algorithms that distribute the dots according to a high frequency pattern, also called xe2x80x9cbluexe2x80x9d noise. U.S. Pat. No. 5,214,517 to Sullivan et al. discloses a digital halftoning method using blue noise to obtain an output image with minimum visual noise. It is understood by those skilled in the art that random noise patterns (a.k.a xe2x80x9cwhitexe2x80x9d noise) contain information at all frequencies, and are therefore visually more objectionable than blue noise patterns.
Minimizing visual noise in digital halftoning by using blue noise dithering can be extended to images with multiple colorant channels, as disclosed in U.S. Pat. No. 5,822,451 to Spaulding et al., which teaches a method of jointly optimizing blue noise dither matricies for each colorant channel to minimize a visual cost function.
It is an object of the present invention to provide for producing high quality digital images that are free of the above described artifacts associated with using excessive amounts of colorant.
It is a further object of the present invention to reduce the amount of colorant used to print an image utilizing a blue noise dithering process to determine which pixels to remove colorant from, which results in improved image quality relative to the prior art.
Another object of the present invention is that the hue of a given color in the input image will not be significantly affected by reducing the amount of colorant to an acceptable level according to the invention.
Yet another object of the present invention is to provide for preferential removal of colorant from specified colorant channels.
These objects are achieved by a method for modifying an-input image having an (x,y) array of input pixels, each pixel having an input colorant amount for one or more color channels to form an output image with output colorant amounts, said output image being suitable for printing on a digital printing device having two or more printing levels, comprising the steps of:
a) determining an average total pixel colorant amount in a neighborhood of an input pixel in the input image;
b) determining a colorant reduction amount for each color channel of the input pixel responsive to the input colorant amount, the average total pixel colorant amount, and a threshold colorant amount;
c) determining an output colorant amount for each color channel of the input pixel responsive to the colorant reduction amount, the input colorant amount, and a spatially periodic dither signal, so that the average output colorant amount in a neighborhood of each pixel in the output image is constrained to be less than the threshold colorant amount; and
d) repeating steps a) through c) for each pixel in the input image.
The present invention has an advantage over the prior art in that it provides for reducing the amount of colorant required to print a digital image without introducing random noise into the image. The present invention utilizes a blue noise dithering process to determine the pixels from which colorant should be removed, which results in blue noise patterns that are visually less objectionable than random noise patterns. The use of a blue noise dithering process when determining the pixels from which colorant should be removed also provides a means to correlate the colorant reduction across the color separation channels, further reducing the objectionability of the noise and thus improving the image quality.